Various techniques for discharging a small quantity of liquid material in the form of a droplet by employing a plunger reciprocating within a liquid chamber, which is in communication with a nozzle, have been proposed up to now. An actuator utilizing a motor, air, a piezoelectric element, for example, is used in many cases as a drive source for moving the plunger. Regarding a discharge device utilizing air pressure as the drive source, Patent Document 1, for example, proposed by the applicant discloses a discharge device in which a discharge port is opened when a plunger rod is moved backward with the aid of air pressure, and in which a droplet is discharged through the discharge port when the plunger rod is moved forward with the aid of resilient force of a spring.
In the discharge device of the type reciprocating the plunger by the spring and the air pressure, a desired distance through which the plunger is movable can be easily obtained, but it is difficult to hold a reciprocating speed of the plunger to be equal to or faster than a certain value because air has compressibility. On the other hand, in a discharge device of the type employing the piezoelectric actuator, because the operation of the piezoelectric element can be controlled with an electrical pulse signal, good reproducibility of a plunger stroke is ensured, and control of the operation of the piezoelectric element is easy.
Regarding a discharge device in which a needle is reciprocated by employing the piezoelectric actuator as the drive source, Patent Document 2, for example, discloses a droplet discharge device comprising a liquid chamber which is in communication with a discharge port and to which a liquid material is supplied, a needle having a tip portion that is moved back and forth in the liquid chamber, a drive device operating the needle to be moved back and forth, and a displacement increasing mechanism, the droplet discharge device discharging a droplet in a form flying out from the discharge port, wherein the drive device is constituted by an even number of drive devices arranged in a bilaterally symmetric relation, the displacement increasing mechanism includes a resiliently deformable U-shaped member having a lower portion to which the needle is linked, the needle is moved backward when the drive devices apply forces acting to move both end portions of the U-shaped member away from each other, and the needle is moved forward when the drive devices apply forces acting to move both the end portions of the U-shaped member in directions coming closer to each other.
In the droplet discharge device using the needle (plunger), since large ejection force can be applied to the liquid material by the plunger that is moved forward at a high speed, it is possible to discharge, in the form of a droplet, even such a liquid material as having high viscosity, which cannot be discharged with an ink jet device using a piezoelectric (electrostrictive) element to push and discharge ink in an ink chamber.
Patent Document 3 discloses a droplet ejection device comprising a casing that has an ejection hole formed at a tip thereof and a cylinder bore, a multilayered piezoelectric element that is arranged in the casing, and a plunger that is driven by the piezoelectric element serving as a drive source, and that is accommodated in the cylinder bore to be able to extend and contract through very small strokes, wherein the piezoelectric element is in the form of a rectangular parallelepiped that is fitted and attached integrally to an element holder, a resilient portion having a thin wall is formed in a part of the element holder to give the piezoelectric element with restoration force when the piezoelectric element is on the contraction side, an upper end portion of the element holder is fixed to the casing, and the plunger is formed at a lower end of the element holder.
The device disclosed in Patent Document 3 includes the plunger having the same diameter as the cylinder bore, and operates on the basis of the discharge principle of discharging the liquid material in the same quantity as the volume of the cylinder bore, which is reduced with forward movement of the plunger. The device operating on the basis of the above-mentioned discharge principle is not suitable for high-speed continuous discharge of several hundred or more shots per second for the reason that sliding friction is caused between a lateral peripheral surface of the plunger and an inner peripheral surface of the cylinder bore.